The present invention relates to an instrument and method for measuring the duration of exposure to temperatures at or above a predetermined temperature, such as a solder reflow temperature. The instrument and method are useful, for instance, for determining duration of a molten state for an integrated circuit assembly during a manufacturing process, such as a solder reflow process.
Interconnection and packaging related issues are among the main factors that determine not only the number of circuits that can be integrated on a chip, but also the performance of the chip. These issues have increased in importance as advances in chip design have led to reductions in the sizes of features on transistors and enlargements in chip dimensions. Industry has come to realize that merely having a fast chip will not result in a fast system; it must also be supported by equally fast and reliable packaging.
Essentially, packaging supplies the chip with signals and power, and performs other functions such as heat removal, physical support and protection from the environment. Another important function of the package is simply to redistribute the tightly packed I/Os off the chip to the I/Os of a printed wiring board.
An example of a package-chip system is the xe2x80x9cflip-chipxe2x80x9d integrated circuit mounted on an area array organic package. Flip-chip mounting entails placing solder bumps on a die or chip, flipping the chip over, aligning the chip with the contact pads on a package substrate, and reflowing the solder balls in an oven to establish bonding between the chip and the substrate. This method is advantageous in certain applications because the contact pads are distributed over the entire chip surface rather than being confined to the periphery, as in wire bonding and most tape-automated bonding (TAB) techniques. As a result, the maximum number of I/O and power/ground terminals available can be increased, and signal and power/ground interconnections can be more efficiently routed on the chips. With flip-chip packaging, proper heating of the chip and the package is essential to ensure proper operation of the final assembly.
It is known in the prior art to package plural discrete integrated circuit components on a single package substrate. For instance, a package may comprise several chips or dice, capacitors, resistors, diodes, etc. It is also known that various integrated circuit components have widely varying heat capacities and coefficients of heating. For instance, small components may heat very quickly, whereas larger components may heat relatively slowly. It is also known that various components have different tolerances to heat. For example, smaller components may be more susceptible to thermal breakdown due to their tendency to heat more quickly than larger components. Thus, it remains a problem in the art that, when packaging several different components on the same package substrate, smaller or more thermally sensitive components may be unduly stressed at temperatures that do not adversely affect larger and less thermally sensitive components.
Moreover, it is not possible to determine heat tolerances of various integrated circuit components except by empirical tests. In such an empirical test, a number of components of the same type are subjected to heating for various lengths of time (holding oven temperature constant), or at various oven temperatures (holding time constant), or both. The results of such empirical testing are data relating to the heat tolerance of the component. However, this type of empirical test does not provide other valuable information relating to the packaging of integrated circuit packages, such as the length of time that the component spends at and above a particular temperature. This information is not provided by the above-described test, because the temperature of the oven at a particular time is not necessarily the temperature in the proximity of the component, as it takes some time for a component""s temperature to reach equilibrium with the oven temperature. Accordingly, an oven may be heated to a particular temperature, such as a solder reflow temperature, but one or more component may not achieve that temperature, or may achieve that temperature only briefly. As a result, manufacturers often find that a part is tolerant to heating at a particular oven temperature for a particular amount of time, only to find later, during production, that this particular combination of oven temperature and time are insufficient to achieve bonding of the component to a package substrate. Moreover, during production the indicated oven temperature generally differs more or less from the actual oven temperature. This phenomenon is at least in part due to the cumulative effect of having several components, all of which are absorbing heat at various rates, in the oven at once. Therefore it remains a problem in the art that it is not currently practical to confirm that a package assembly comprising a plurality of components that is introduced into a reflow oven will reach and exceed a particular temperature, such as a solder reflow temperature, for an appropriate length of time.
There is therefore a need in the art for an instrument that will measure the duration of heating of an article, component or assembly of components a temperatures equal to and greater than a predetermined temperature, such as a solder reflow temperature. There is also a need in the art for a method employing such an instrument for determining the length of time that and article to be heated, such as an integrated circuit package, is at temperatures equal to and above a predetermined temperature, such as a solder reflow temperature.
The present invention meets the need in the art for an instrument that will measure the duration of heating an article, component or assembly of components at temperatures equal to and greater than a predetermined temperature, such as a solder reflow temperature. An instrument according to the present invention comprises a capillary tube having an opening and an indicator material, such as a meltable solid, which has a melting point at the predetermined temperature. The indicator material is adjacent and in contact with the capillary tube opening. When an instrument according to the present invention is heated to a temperature equal to or above the predetermined temperature, the indicator material melts and begins to move into the capillary tube by capillary action at a time-dependent rate, such as a linear rate. When the instrument is cooled below the predetermined temperature, the indicator material solidifies and remains in the capillary tube. The amount of indicator material present in the capillary tube is related to the amount of time the instrument spent at temperatures at or above the melting point of the indicator material. The duration of heating of an instrument according to the present invention at temperatures equal to and greater than a predetermined temperature is then indicated by the instrument of the present invention.
The present invention meets a need in the art for a method of empirically determining the duration of actual temperatures at or above a predetermined temperature, such as a solder reflow temperature. In a method according to the present invention, an instrument according to the present invention is placed in proximity to an article or articles to be heated, heated along with the article or articles, and then cooled. The amount of indicator material in the capillary tube is used to determine the duration of the indicator material at or above the predetermined temperature.
The present invention also meets a need in the art for an instrument and a method of measuring and indicating duration of actual temperatures within a heating device, such as a reflow oven, at temperatures at and above a predetermined temperature, such as a solder reflow temperature. An instrument according to the present invention is small, on the order of 1-10 cm in length, and thus may be placed close to an article to be heated within the heating device. This permits convenient measurement of duration of heating at temperatures equal to and greater than a predetermined temperature, such as a solder reflow temperature.
Additional advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.